/* Copyright (c) 2011 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.appengine.demos.shardedcounter.java.v2;

import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.EntityNotFoundException;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Transaction;
import com.google.appengine.api.memcache.Expiration;
import com.google.appengine.api.memcache.MemcacheService;
import com.google.appengine.api.memcache.MemcacheService.SetPolicy;
import com.google.appengine.api.memcache.MemcacheServiceFactory;

import java.util.Random;

/**
 * A counter which can be incremented rapidly.
 * 
 * Capable of incrementing the counter and increasing the number of shards. When
 * incrementing, a random shard is selected to prevent a single shard from being
 * written too frequently. If increments are being made too quickly, increase
 * the number of shards to divide the load. Performs datastore operations using
 * the low level datastore API.
 */
public class ShardedCounter {

  /**
   * Convenience class which contains constants related to a named sharded
   * counter. The counter name provided in the constructor is used as
   * the entity key.
   */
  private static final class Counter {
    /**
     * Entity kind representing a named sharded counter.
     */
    private static final String KIND = "Counter";

    /**
     * Property to store the number of shards in a given {@value #KIND} named
     * sharded counter.
     */
    private static final String SHARD_COUNT = "shard_count";
  }

  /**
   * Convenience class which contains constants related to the counter shards.
   * The shard number (as a String) is used as the entity key.
   */
  private static final class CounterShard {
    /**
     * Entity kind prefix, which is concatenated with the counter name to form
     * the final entity kind, which represents counter shards.
     */
    private static final String KIND_PREFIX = "CounterShard_";

    /**
     * Property to store the current count within a counter shard.
     */
    private static final String COUNT = "count";
  }

  private static final DatastoreService ds = DatastoreServiceFactory
      .getDatastoreService();

  /**
   * Default number of shards.
   */
  private static final int INITIAL_SHARDS = 5;

  /**
   * The name of this counter.
   */
  private final String counterName;

  /**
   * A random number generating, for distributing writes across shards.
   */
  private final Random generator = new Random();

  /**
   * The counter shard kind for this counter.
   */
  private String kind;

  private final MemcacheService mc = MemcacheServiceFactory
      .getMemcacheService();

  /**
   * Constructor which creates a sharded counter using the provided counter
   * name.
   * 
   * @param counterName name of the sharded counter
   */
  public ShardedCounter(String counterName) {
    this.counterName = counterName;
    kind = CounterShard.KIND_PREFIX + counterName;
  }

  /**
   * Increase the number of shards for a given sharded counter. Will never
   * decrease the number of shards.
   * 
   * @param count Number of new shards to build and store
   * @return the new number of shards
   */
  public long addShards(int count) {
    Key counterKey = KeyFactory.createKey(Counter.KIND, counterName);
    return incrementPropertyTx(counterKey, Counter.SHARD_COUNT, count,
        INITIAL_SHARDS + count);
  }

  /**
   * Retrieve the value of this sharded counter.
   * 
   * @return Summed total of all shards' counts
   */
  public long getCount() {
    Long value = (Long) mc.get(kind);
    if (value != null) {
      return value;
    }

    long sum = 0;
    Query query = new Query(kind);
    for (Entity shard : ds.prepare(query).asIterable()) {
      sum += (Long) shard.getProperty(CounterShard.COUNT);
    }
    mc.put(kind, sum, Expiration.byDeltaSeconds(60),
        SetPolicy.ADD_ONLY_IF_NOT_PRESENT);

    return sum;
  }

  /**
   * Increment the value of this sharded counter.
   */
  public void increment() {
    /* Find how many shards are in this counter. */
    int numShards = getShardCount();

    /* Choose the shard randomly from the available shards. */
    long shardNum = generator.nextInt(numShards);

    Key shardKey = KeyFactory.createKey(kind, Long.toString(shardNum));
    incrementPropertyTx(shardKey, CounterShard.COUNT, 1, 1);
    mc.increment(kind, 1);
  }

  /**
   * Get the number of shards in this counter.
   * 
   * @return shard count
   */
  public int getShardCount() {
    try {
      Key counterKey = KeyFactory.createKey(Counter.KIND, counterName);
      Entity counter = ds.get(counterKey);
      Long shardCount = (Long) counter.getProperty(Counter.SHARD_COUNT);
      return shardCount.intValue();
    } catch (EntityNotFoundException ignore) {
      return INITIAL_SHARDS;
    }
  }

  /**
   * Increment datastore property value inside a transaction. If the entity with
   * the provided key does not exist, instead create an entity with the supplied
   * initial property value.
   * 
   * @param key the entity key to update or create
   * @param prop the property name to be incremented
   * @param increment the amount by which to increment
   * @param initialValue the value to use if the entity does not exist
   * @return the new value
   */
  private long incrementPropertyTx(Key key, String prop, long increment,
      long initialValue) {
    Transaction tx = ds.beginTransaction();
    Entity thing;
    long value;
    try {
      thing = ds.get(tx, key);
      value = (Long) thing.getProperty(prop) + increment;
    } catch (EntityNotFoundException e) {
      thing = new Entity(key);
      value = initialValue;
    }
    thing.setUnindexedProperty(prop, value);
    ds.put(tx, thing);
    tx.commit();
    return value;
  }
}
